1. Field of the Discussed Inventive Concept(s)
The presently disclosed and claimed inventive concepts are generally within the field of coagulation diagnosis and relate to an in vitro method for determining von Willebrand factor (VWF) activity in a sample. A discussed method includes the use of a mutant gain-of-function variant of the GPIbα protein, thereby dispensing with the need to use ristocetin, botrocetin or another ristocetin- or botrocetin-equivalent substance. The presently disclosed and claimed inventive concept(s) further relate to a disclosed method for determining von Willebrand factor (VWF)-cleaving activity of ADAMTS-13 protease.
2. Relevant Background Information
Von Willebrand factor (VWF) is a high molecular weight, multimeric glycoprotein found in blood plasma, and functions in the process of primary hemostasis. VWF possesses, inter alia, binding sites for collagen and for glycoprotein Ib (GPIb) which is located on the surface of thrombocytes. GPIb is an integral membrane protein which, together with another integral membrane protein, glycoprotein IX (GPIX), forms the glycoprotein Ib-IX-receptor complex in the thrombocytic membrane. GPIb is a two-chain molecule comprising a heavy chain having an apparent molecular mass of about 145 kDa (synonyms: alpha-chain or GPIbα) and a light chain having an apparent molecular mass of about 22 kDa (synonyms: beta-chain or GPIbβ), which are linked to one another by disulfide bonds [Lopez, J. A. et al. (1987) Cloning of the a chain of human platelet glycoprotein Ib: A transmembrane protein with homology to leucine-rich α2-glycoprotein. Proc. Natl. Acad. Sci. USA 84: 5615-5619, the entire contents of which are hereby incorporated by reference].
In the case of a vessel injury, collagen surfaces are thereby exposed and to which VWF binds. Due to its binding to collagen and under the influence of increased shearing forces acting on the collagen-bound VWF, VWF is altered or activated in such a way that it can bind to the amino-terminal end of the GPIb heavy chain (GPIbα) in the GPIb-IX-receptor complex of the thrombocytic membrane. In this way, the activated VWF captures passing thrombocytes, resulting in the formation of a first agglomeration of VWF, collagen and thrombocytes at the site of the injury. Subsequently, the thrombocytes are activated, thereby also starting plasmatic coagulation which finally, after a plurality of amplifying cascades and attachment of further thrombocytes, results in the wound being closed.
Qualitative or quantitative VWF disorders are the cause of a “von Willebrand syndrome” (synonyms: von Willebrand disease, VWD), one of the most common hereditary hemorrhagic conditions. Various screening methods are available for diagnosing von Willebrand syndrome, such as, for example, but not by way of limitation, methods for determining bleeding time (BT), quantitative methods for determining the VWF antigen concentration (VWF:Ag), such as, for example, ELISA methods; and methods for determining the VWF activity, such as ristocetin-induced platelet agglutination (VWF:RCo).
The method of ristocetin-induced aggregation of stabilized thrombocytes, which is also referred to as ristocetin cofactor assay, also recognizes those functional defects of the VWF protein that are not detected by quantitative methods for determining the VWF antigen concentration. Complete diagnosis of a von Willebrand syndrome therefore requires carrying out a ristocetin cofactor assay for determining ristocetin cofactor activity. The ristocetin cofactor assay is usually carried out by mixing a patient's plasma sample with fixed thrombocytes and with ristocetin. Ristocetin induces binding of the VWF present in the sample to the GPIb receptor of the added thrombocytes, resulting in the latter aggregating. The extent of this aggregation reaction correlates with the amount of active VWF present in the patient's sample. Said aggregation reaction may be recorded optically, for example, by measuring the increase in transmission, thereby enabling the VWF:RCo activity to be quantified.
Compared to a VWF antigen assay, the ristocetin cofactor assay has the advantage of determining the activity of VWF and therefore enables functional VWF disorders and the classification of various subtypes of von Willebrand syndrome, only some of which are also accompanied by a reduced VWF antigen concentration, to be recognized. Said subtypes are often classified by forming the ratio of VWF ristocetin cofactor activity (VWF:RCo) and VWF antigen concentration (VWF:Ag). A VWF:RCoNWF:Ag ratio of less than 1 is characteristic for the von Willebrand syndrome subtypes 2A, 2B and 2M. A frequently recommended threshold is a ratio of 0.7.
A disadvantage of the classical ristocetin cofactor assay is the fact that it is difficult to automate because the thrombocytes in the assay mix must constantly be stirred during the measurement. Another disadvantage is the relative imprecision of the assay and an insufficient determination of small VWF activities in the range of between 0 and 20% of standard variables.
In recent times, GPIbα-based VWF ELISAs have been developed which make use of recombinant GPIbα fragments (1-289 and 1-290, respectively) containing the amino-terminal VWF binding region [e.g., WO 01/02853 A2; Vanhoorelbeke, K. et al. (2002). A reliable von Willebrand factor: Ristocetin cofactor enzyme-linked immunosorbent assay to differentiate between type 1 and type 2 von Willebrand disease; and Semin Thromb Hemost. 28(2): 161-165 and Federici, A. B. et al. (2004) A sensitive ristocetin co-factor activity assay with recombinant glycoprotein Ibα for the diagnosis of patients with low von Willebrand factor levels. Haematologica 89(1): 77-85, the entire contents of each being hereby expressly incorporated by reference]. These assays involve binding a recombinant GPIbα fragment to an ELISA plate with the aid of a specific antibody. Ristocetin is added to the patient's sample in order for the VWF of the patient's sample to be able to bind to the recombinant GPIbα fragment. Finally, the bound VWF is quantitatively detected with the aid of an anti-VWF antibody. This kind of VWF ELISA has been shown to correlate very well with the results of the classical thrombocyte-based VWF ristocetin cofactor activity assay and to be even more sensitive and more precise.
Hui et al. (Abstract, ISTH 2007, the entire contents of which are expressly incorporated herein by reference) describe a GPIbα-based VWF ELISA which makes use of recombinant GPIbα fragments (1-483) having mutations in positions 233 and 239. These mutations are “gain-of-function” mutations which are known to have a higher affinity for VWF in the presence of low ristocetin concentrations and to interact with VWF more strongly than wild-type GPIbα protein (WO 93/16712, the entire contents of which are expressly incorporated herein by reference). The mutations mentioned are well known mutant variants of the GPIbα chain. Substitution of the glycine residue in position 233 of the GPIbα chain by a valine residue (G233V) has been described by Miller et al. (U.S. Pat. No. 5,317,097, the entire contents of which are expressly incorporated herein by reference). Said mutation is the cause of platelet-type von Willebrand syndrome (PT-VWD), an autosomally dominantly inherited hemorrhagic condition. Substitution of the methionine residue in position 239 of the GPIbα chain by a valine residue (M239V) has been described by Russell & Roth [Russell, S. D. & Roth, G. J. (1993) Pseudo-von Willebrand Disease: A mutation in the platelet glycoprotein Ibα gene associated with a hyperactive surface receptor. Blood 81(7), 1787-1791, the entire contents of which are expressly incorporated herein by reference]. This mutation also causes PT-VWD.
Disadvantageously, the GPIbα-based VWF ELISAs as well as the ristocetin cofactor assay are based on using ristocetin or other exogenous, non-physiological modulators which mediate binding of VWF to the GPIbα chain. Ristocetin, an antibiotic glycopeptide of the bacterium Nocardia lurida, and botrocetin (synonym: co-agglutinin), a snake venom of the genus Bothrops, are known to be used for inducing binding of VWF to thrombocytes or to isolated GPIbα protein or fragments thereof in vitro. Using ristocetin has the disadvantage that it can bind not only to VWF but also to many other proteins, for example, but not by way of limitation, fibrinogen and anti-GPIbα antibodies (observed in some experiments). Using ristocetin in VWF assays therefore runs the risk of the occurrence of unspecific binding reactions or precipitation reactions, which may falsify the test result.
Accordingly, disclosed and claimed herein is at least one method for determining VWF activity, which can be carried out in the absence of ristocetin. One example of the disclosed and claimed method(s) is achieved by using a gain-of-function variant of the GPIbα chain, which has two point mutations, in position 233 and in position 239, as compared to the amino acid sequence of human GPIbα receptor. A gain-of-function variant of the GPIbα chain, as disclosed and claimed, has a significantly higher affinity for VWF and interacts with VWF more strongly than wild-type GPIbα protein in the presence of low ristocetin concentrations.
ADAMTS-13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, member 13) is a metalloprotease which proteolytically cleaves von Willebrand factor (VWF), thereby reducing its activity. Congenital and acquired deficiencies in ADAMTS-13 enzyme activity are known, which cause various diseases such as, for example, thrombotic, thrombocytopenic purpura (TTP), a life-threatening thromboembolic disorder affecting microcirculation. Unduly large multimers of VWF are found in the plasma of TTP patients and are considered to be the cause of the formation of thrombi rich in VWF and thrombocytes. Determination of the VWF-cleaving protease activity of ADAMTS-13 protease in patients' samples is therefore of diagnostic importance.
Various methods for determining the VWF-cleaving activity of ADAMTS-13 protease and for diagnosing ADAMTS-13 deficiency have been disclosed. Furlan et al. (1996) describe a method, wherein a sample containing ADAMTS-13 protease is incubated with purified human VWF as substrate, with the proteolytic activity of the ADAMTS-13 protease being detected by subsequent analysis of VWF multimers by means of SDS agarose gel electrophoresis or by analyzing VWF fragments by means of SDS polyacrylamide gel electrophoresis (SDS PAGE) and subsequent immunoblotting [Furlan, M., Robles, R. and Lämmle, B. (1996) Partial purification and characterization of a protease from human plasma cleaving von Willebrand factor to fragments produced by in vivo proteolysis. Blood 87, 10: 4223-4234, the entire contents of which are hereby expressly incorporated by reference.] Multimer analyses by means of gel electrophoresis and immunoblotting require an extraordinary amount of work and time and are therefore not suitable for use in a clinical laboratory.
Other functional assay methods determine the proteolytic activity of ADAMTS-13 protease by incubating the sample containing said ADAMTS-13 protease with VWF as substrate and then determining the loss of activity of said VWF substrate. The more ADAMTS-13 protease present in the sample, the more VWF substrate is cleaved and the less VWF activity can then be detected in the reaction mixture. The advantage of these functional methods is that of simulating in vitro the in vivo-relevant function of ADAMTS-13 protease by using VWF substrate and measuring the loss of function of the large multimers. For example, WO 2004/005451 A2 (the entire contents of which are hereby expressly incorporated by reference) discloses a method, wherein a sample containing the ADAMTS-13 protease is incubated with purified human VWF substrate, with the proteolytic activity of said ADAMTS-13 protease being detected by subsequently determining the VWF activity remaining in the reaction mixture by means of a ristocetin cofactor assay. Alternatively, the VWF activity remaining in the reaction mixture may be assayed by means of a collagen binding assay (see, for example, WO 00/50904, the entire contents of which are hereby expressly incorporated by reference). Disadvantageously, the reaction mixture must be incubated for a very long time, sometimes overnight, in order to cause sufficient proteolytic degradation of VWF. An improved method with a reduced incubation time of 60 minutes is described in Kostousov et al. (Kostousov, V., Fehr, J., Bombeli, T. (2006) Novel, semi-automated, 60-min-assay to determine von Willebrand factor cleaving activity of ADAMTS-13. Thrombosis Res. 118: 723-731, the entire contents of which are hereby expressly incorporated by reference.)
US 2007/0065895 A1 and US 2005/0186646 A1 (the entire contents of both of which are hereby expressly incorporated by reference) disclose methods in which a sample containing ADAMTS-13 protease is incubated with peptides having a specific cleavage site for ADAMTS-13 protease or with VWF peptides (fragments) as substrate, with the proteolytic activity of ADAMTS-13 protease being detected by subsequently analyzing the resulting peptide fragments by means of SDS PAGE and optionally by subsequent immunoblotting.
The known assay systems are disadvantageous in that they either require complicated technologies, such as, for example, analysis of the substrate fragments resulting from proteolysis by means of SDS PAGE, or do not use the natural VWF substrate, thereby making it possible that some dysfunctionalities of ADAMTS-13 protease cannot be detected.
The presently disclosed and claimed inventive concept(s) provide a method for determining von Willebrand factor (VWF)-cleaving activity of ADAMTS-13 protease. The disclosed and claimed method(s) enable as many dysfunctionalities of ADAMTS-13 protease as possible to be detected. Furthermore, the presently disclosed and claimed method(s) can be carried out in a timesaving manner and automatically on conventional clinical analyzers. One such exemplary method comprises mixing a sample which is suspected to contain ADAMTS-13 protease with isolated VWF as substrate to give a reaction mix. The VWF substrate is a high molecular weight, multimeric VWF which includes VWF monomers having at least one amino acid sequence variation (polymorphism or mutation) resulting in each case in the VWF substrate being broken down by ADAMTS-13 in an accelerated manner compared to a VWF substrate that is composed of wild-type VWF monomers.